Optical strain gauges are known in the most varied forms and usually consist of a film-type carrier layer or are cast into such a carrier layer. These carrier layers have a bottom support or substrate on which an optical waveguide is fastened or secured, which includes a section with a fiber Bragg grating. By such a fiber Bragg grating, a coherent light beam fed into the optical waveguide is reflected with a certain wavelength, and can be detected as a reflection peak. If such an optical strain gauge is applied onto a deformation body, thereby due to a strain the wavelength of the reflection peak will vary proportionally to the strain. Such optical strain gauges are therefore usable similarly as electrical strain gauges with a resistance grid, and can be used for the detection of the most varied physical quantities or values.
In practice it has been noted, that such reflection peaks comprise several reflection maxima or a maximum that is considerably enlarged in width, due to mechanical tensions orthogonal to the fiber direction or due to transversely directed strain fields, whereby such maxima or such an enlarged maximum cannot be exactly resolved with an unambiguous wavelength in current evaluating devices.
Such an optical strain gauge for the measurement of mechanical tensions is known from the EP 1 129 327 B1. That relates to an optical strain gauge that is embodied as a fiberoptic rosette. Thereby the optical strain gauge consists of a carrier or support material that is embodied as a rigid flat plate, onto which an optical waveguide with three sections with respectively one fiber Bragg grating is glued, and which is covered by a further flat glued-on plate. In a different embodiment, the optical waveguide is encapsulated in a hard carrier or support material of a cured epoxy resin. For the strain measurement, the optical strain gauge with its plate-shaped bottom support or a bottom support consisting of an epoxy resin layer is glued onto a deformation or strain body, of which the strain is to be detected. For the adhesive glued mounting, the optical strain gauge with its bottom support must be tightly or fixedly pressed onto the deformation or strain body, whereby already strong transverse forces are introduced into the rigid epoxy resin layer or the plates. Due to the rigid connection with the fiber Bragg grating, then a remaining residual tension or stress can remain in the fiber Bragg grating, which often leads to a strong spreading or widening of the reflection peak. Such optical strain gauges with their fiber Bragg gratings encapsulated in epoxy resin or the fiber Bragg gratings glued between two hard plates can also, however, be fixedly inserted in carbon fiber reinforced composite materials or cement materials for the strain determination, whereby transverse forces, which partially also remain, are introduced into the fiber Bragg grating during the curing process. This then often leads to interferences in the fiber Bragg gratings, which lead to strong spreading or widening of the reflection peaks with one or several maxima, of which the reflection wavelengths can then only be detected sufficiently accurately with difficulty.
However, from the DE 196 48 403 C1, an optical sensor transducer for the detection measurement with an integrated fiber Bragg grating is known, in the fiber Bragg grating of which no transverse forces can be introduced, which could lead to a spreading or widening of the reflection peaks. This sensor transducer involves a force or load transducer with which both tension as well as compression forces can be detected. Therefore the optical waveguide is arranged with its fiber Bragg gratings between two spaced-apart clamping elements, which are pre-tensioned in the tension direction against one another by a compression spring and a strain body. Thereby the optical waveguide with its fiber Bragg grating section is arranged in the pipe-shaped strain body and is secured in a non-positive frictional or force-transmitting manner on the clamping elements. Apparently a soft filler material is provided in the area of the fiber Bragg grating in the hollow space between the strain body and the waveguide section with the fiber Bragg grating. For the force measurement, the force is introduced, in tension or compression, into the two clamping elements, whereby the waveguide section with the fiber Bragg grating can be expanded or extended as well as compressed or upset, and therewith its reflection wavelength changes in both directions proportionally to the force introduction. Due to the protected installation of the fiber Bragg grating section, the reflection peaks remain relatively narrow, so that the wavelength change is exactly detectable. However, such pipe-shaped transducer elements with the clamping elements provided perpendicularly thereto for the force introduction are very voluminous and complicated in the production. Moreover, such transducer elements also can only be secured with difficulty on the strainable surfaces that are to be detected. Still further, a portion of the strain measurement range in the tension direction is already lost due to the pre-tensioning, so that thereby larger strain effects are no longer detectable.